I understand that's it's easy to crash the economy. It's easy enough to do this without leasing in any case 
But once you knwo what you're doing (which can still take some time), will leasing not become much more viable? For two reasons: if you can lease your way into winning, then who cares about your economy (this assumes a small enough map or endgame situation where you're able to finish the game before the pain that comes from lease costs start to matter). And if you can postpone payment to a moment when you can afford it, then leasing can be the correct decision if you can gain what you need that much faster by a greater quantity.
In the first case, you're sacrificing your long term viability completely because there is no long term (i.e. all or nothing play). It may be cheesy, but it should work in certain circumstances.
In the second case, you're sacrificing part of your mid term income for a short term gain. The assumption is that without the lease, a rush is not affordable (or you're not able to afford other expenses). The expectation is that the short term gain is great enough to offset the mid term loss of income. An extreme example would be longterm leases on several colony ships on a map with several high quality planets in a border region and direct purchase of the colony ships would be prohibitively expensive.
Total cost is certainly not the only or perhaps not even the main consideration in leasing. The moment of payment is perhaps a much more important consideration for leasing.
I think once I've gotten the economic model mastered a little bit, I'll start playing about with leases on a more serious level. I'll use it as an investment loan. Let's see if that will make me flourish or crash and burn like banks 